msm: pil-msa: Don't unconditionally write to restart_reg

Some targets don't have a top level MSS_RESTART register.
Fix the error path in pil_msa_pbl_reset to check if restart_reg
has been mapped in before writing to it.

Change-Id: I498e9e4f5aba9792e8524b0b3053c653b802fe63
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>